Pentadienyl anion stability

From their analysis of the conformational energies of pentadienyl anion and the penta-dienyl metal compounds, Pratt and Streitwieser in 2000 pointed out that the stabilization of the planar forms of the organometallic structures results from both conjugation and electrostatic attraction between the negative carbons and the alkali metal cations. To determine the relative magnitude of these effects, the reaction energies were determined for hypothetical reaction, shown in Scheme 1 where M represents any alkali metal. 

Non-planarity is the result of the dominance of the destabilizing interactions of the sulfur lone pair and tt- occupied MOs of the pentadienyl anion over the stabilizing interaction of that lone pair and the LUMO of the anion fragment. In fact thiabenzene is antiaromatic in a planar configuration. Pyramidalization reduces the antiaromaticity induced by the sulfur. Although no X-ray data are available on the parent system, kinetic data have been obtained supporting a minimum barrier to inversion at the pyramidal sulfur of a 2-thianaphthalene of 99.1 kJ mol-1 (75JA2718). The formulation of the system as a cyclic ylide is supported by the chemical reactivity of the compounds as related in the reactivity section below. 

Efficient methods for the preparation of pentadienyl compounds of the alkali metals have now been developed. Treatment of 1,4-dienes with butyllithium in the presence of tetrahydrofuran (thf) at —78° yields deep orange solutions which contain pentadienyllithiums (36,37). Any excess butyllithium may be destroyed by its reaction with thf by allowing the mixture to warm up briefly to room temperature (38). Similar results are obtained using potassium amide in liquid ammonia (39). 1,3-Dienes, however, do not yield pentadienyl anions under these conditions, unless the diene is conjugated with a phenyl (40), vinyl (41), trimethylsilyl (48), or similar stabilizing group. Unfortunately, 1,4-dienes are not very readily accessible. However, 1,3- as well as 1,4-dienes can be metallated using a 1 1 mixture of butyllithium and potassium tm-butoxide (49). Trimethylsilylmethylpotassium is also effective (44). 

Apparently, the pentadienyl anion portion moved along the carbon atom backbone until a heptatrienyl anion was formed, which then underwent the expected cyclization (see Sect. C.I) to a heptadienyl anion, which is of greater stability due to its extra carborn carbon single bond. Finally, while the 3-methylcycloheptadienyI anion was found to be thermally stable, it could be completely converted to the 2-methylcycloheptadienyl anion photochemically. 

However, another aspect of these methyl groups pertains to whether or not they serve to stabilize or destabilize a pentadienyl anion. Of course, it is clearly recognized that a stabilizing influence would ensue for pentadienyl cations and radicals. Results of the MNDO study indicate that methylation at an active (i.e., 1,5, or especially 3) position of a pentadienyl anion is accompanied by a stabilizing effect of ca. 1 kcal/mol, while methylation at a 2 or 4 site is destabilizing, by ca. 2. kcal/mol188. For the pentadienyl cation, methylation of the 1, 2, or 3 positions always leads to stabilization, by ca. 6, 1, and 4.5 kcal/mol, respectively. 

Hiickel pointed out that, on the basis of molecular orbital theory, monocyclic conjugated polymethines have filled shells of tt-electrons when the number of TT-electrons is An + 2, where n is an integer. These systems may be expected to be stable. The rule may be illustrated by reference to Fig. 2.1. If = 0, then a system with 27r-electrons should be stable. Such a situation is found in the cyclopropenyl positive ion, which has been isolated as the hexachloroanti-monate. For n = the prediction is that the cyclopentadienyl anion, benzene and the cycloheptatrienyl (tropylium) cation are stable. This is certainly in accord with experience. The stability of benzene is well known, the cydo-pentadienyl anion is readily formed by the action of potassium metal on cyclopentadiene, and the cycloheptatrienyl cation is one of the most stable carbonium ions known. Huckel s rule also predicts that some of the larger cyclic conjugated systems are stable, e.g. those with 10,14 and 18 rr-electrons. However, the situation is complicated by steric problems (see for example Garratt, 1971) and need not be considered further here. 

In iron reactions where the reagent was equivalent to C, described in Section 8.9.A, the iron moiety was used as an auxiliary. Iron can also stabilize cations, which then react with nucleophiles to generate new carbon-carbon bonds. 08 xhese cations are formed as iron-alkene complexes, usually by reaction of cyclo-pentadienyl dicarbonyl ferrate anion (478) with an allylic halide such as 3-chloro-2-methyl-l-propene. The... 

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